How Do You Move About in Space?
A spacecraft uses most of its energy getting up into space. With no drag to overcome once in orbit, the spacecraft can then “coast” without expending any more energy. But how can you change a spacecraft’s course? Move it from one orbit to another? Or bring it back to Earth?
View of the Space Shuttle Discovery as photographed during the survey operation performed by the crew on the International Space Station. As part of the survey and part of every mission's activities, the orbiter performed a back-flip for the rendezvous pitch maneuver.
How do Rockets Help You Maneuver in Orbit?
Speeding up and slowing down in orbit works just opposite to what you might expect. The larger a spacecraft's orbit, the slower the spacecraft travels. So if you wanted to pass a spacecraft just ahead of you, you would have to fire a thruster in a forward direction. This would decrease your orbital energy and drop you into a lower orbit, where you would travel faster! The "passing lane" in orbit is always lower.
How do Rockets Help You Maneuver in Open Space?
Once you are far from a planet, say, while flying between Earth and Jupiter, mid-course corrections are fairly straight-forward. To speed up, you fire a rear-facing thruster. To slow down, you fire a forward-facing thruster. To alter your course, you fire a thruster in a sideward direction. To rotate your spacecraft, you fire a pair of sideward-pointed thrusters located near opposite sides of the spacecraft. To stop rotating, you fire thrusters aimed in the opposite direction.